Freeze drying is a well known process used to prepare storage stable formulations of pharmaceutical compounds which otherwise suffer degradation when stored in the presence of water, for example, because of disproportionation and/or hydrolysis. A typical freeze drying cycle consists of four stages. Freezing the composition of the compound to be freeze dried, a primary drying cycle which comprises applying a vacuum and sufficient heat to sublimate the ice present in the composition, a second drying cycle which removes any residual water and then recovery of the freeze dried composition. It is an expensive process because it takes a long time and because a low temperature and a vacuum are required. A low temperature is required because the vacuum needs to be applied at a temperature below the eutectic temperature for mixtures of crystalline substances or below the glass transition or collapse temperature for amorphous mixtures. This is to ensure that the water present is vapourised without passing through the liquid state and so that the amorphous mixtures do not collapse. A collapsed amorphous mixture is effectively useless because it is very difficult to reconstitute and may be unstable.
To keep costs down it is preferable for the collapse or eutectic temperature not to be too low in order that the cooling cost is reduced. A higher collapse or eutectic temperature is also advantageous because the evaporation is hastened which reduces the length of time the vacuum is needed. Compositions suitable for freeze drying have been sought which produce a stable product and for which the collapse or eutectic temperature is not too low.